The invention relates to color selection means for a cathode-ray tube (CRT) and, particularly, to a color selection structure having a metal masking plate with strips of insulation disposed on one surface of the masking plate. A potential is applied to the masking plate, and a focusing potential is developed on the strips of insulation by secondary and reflected primary electrons emanating from a display screen and incident on the insulation.
A conventional shadow-mask-type color television picture tube, which is a CRT, includes generally an evacuated envelope having therein a target comprising an array of phosphor elements of three different emission colors arranged in color groups in cyclic order, means for producing three convergent electron beams directed toward the target, and a color selection structure having an apertured masking plate closely spaced from the target. The masking plate shadows the target, and the differences in convergence angles permit the transmitted portions of each beam to select and excite phosphor elements of the desired emission color.
At about the center of the color-selection structure, the masking plate of a commercial CRT intercepts all but about 18% of the beam currents; that is, the plate is said to have a transmission of about 18%. Thus, the area of the apertures of the plate is about 18% of the area of the masking plate. Since there are no focusing fields present, a corresponding portion of the target is excited by the beamlets of each electron beam.
Several methods have been suggested for increasing the transmission of the masking plate; that is, increasing the area of the apertures relative to the area of the plate, without substantially increasing the excited portions of the target area. In one approach, each of the apertures of the color-selection structure is defined by a quadrupolar electrostatic lens which focuses the beamlets passing through the lens in one transverse direction and defocuses them in the orthogonal transverse direction on the target depending upon the relative magnitudes and polarities of the electrostatic fields comprising the lens.
In one type of quadrupolar-lens color-selection structure described in U.S. Pat. No. 4,059,781 to W. M. van Alphen et al., a strong focusing quadrupolar lens is generated from voltages applied between two sets of substantially-parallel conducting strips, each set being orthogonally positioned with respect to the other, and insulatingly bonded at the intersections of the strips.
In a second approach to increasing the transmission of the masking plate, each aperture in the masking plate is enlarged and split into two adjacent windows by a conductor to form a dipole-quadrupolar lens. The two beamlets passing through the windows of each aperture are deflected around the conductor towards one another, and both beamlets fall on substantially the same area of the target. In this second approach, the transmitted portions of the beams are also focused in one transverse direction and defocused in the orthogonal transverse direction.
One type of dipole-quadrupolar lens color selection structure is described in West German Offenlegungsschrift No. 2,814,391, published Oct. 19, 1978. That publication discloses a CRT having a target, as normally viewed, comprised of a mosaic of vertical phosphor stripes of three different emission colors arranged cyclically in triads (groups of three different stripes), means for producing three convergent horizontally in-line electron beams directed towards the target, and a color selection structure located adjacent the target. The color selection structure comprises a metal masking plate having therein an array of substantially square apertures arranged in vertical columns, and an array of narrow vertical conductors insulatingly spaced from the masking plate, with each conductor substantially centered over the apertures of each of the columns of apertures. Each aperture is also centered over a triad of phosphor stripes. Viewed from the electron-beam-producing means, the conductors divide each aperture into two essentially-equal horizontally-coadjacent windows. This prior art color selection structure has windows with a width-to-height ratio of about 0.46 and transmits about 44% or less of the electron beams.
In any of these and similar structures, it is a major problem to construct an array of conductors that are electrically insulated from one another with the insulation not exposed to the electron beams as is required in a CRT color selection structure. Impingement of the electron beams on exposed portions of the insulation results in localized charging of the insulation and deflection of the electron beams.
U.S. Pat. No. 4,128,790, issued to Steeghs on Dec. 5, 1978, discloses a CRT having a color selection means which utilizes the electron beams to charge one surface thereof to provide a bias voltage. In the Steeghs patent, the color selection means, shown in FIG. 1, comprises an apertured metal plate having vertical conductive strips disposed between the apertures. The conductive strips are insulated from the metal plate by means of insulating material, for example, a glass layer about 0.06 mm thick. The conductive strips consist of vapor-deposited aluminum about 0.0005 mm thick overlying the glass layer. The conductive strips are mutually interconnected but otherwise insulated from the metal plate. During operation of the tube, a voltage of 25 kV is applied to the metal plate and to the display screen of the tube. The electron beams from the electron guns are partially intercepted by the conductive strips of the color selection means which become negatively charged to a suitable bias voltage. In this tube structure, the metal plate of the color selection means is adjacent to the display screen, and the insulative glass layer and the conductive strips are formed on the side of the metal plate facing the electron gun. The bias potential on the conductive strips is controlled by a voltage stabilization rectifier comprising a series arrangement of twenty Zener diodes, each having a Zener voltage of 75 volts, so that the overall Zener voltage is 1.5 kV. The Zener diodes limit the voltage difference of the color selection means because of the conduction of the diodes, and the impinging electron beams prevent the voltage difference from decreasing because of the negative charge imparted to the conductive strips.
The color selection structure described in the Steeghs patent is complex and expensive to manufacture. The electrical reliability of the structure depends on the electrical integrity of the twenty Zener diodes and on the integrity of the aluminum film strips vapor deposited on the glass layer. Electrical breakdown at one point along the strips would result in complete collapse of the bias voltage. Thus, the need exists for a simple, inexpensive and reliable color selection structure.